Aluminum hydroxide gel particles are used as a pharmaceutical antacid, a flame retardant, or an electric or electronic material in a wide variety of fields. However, since it has a Fe content close to 200 ppm, there is a limit to the use of the particle as an electric or electronic material which is required to have high purity.
To produce aluminum hydroxide, there is known a method in which a sodium aluminate solution is used as a starting material. For example, Patent Document 1 proposes a method for producing aluminum hydroxide through the following steps. That is, an oxidant is caused to act on a sodium aluminate aqueous solution to decompose organic matter contained in sodium aluminate. Thereafter, seed crystal aluminum hydroxide is added to precipitate a small amount of aluminum hydroxide (Fe as an impurity contained in sodium aluminate as a raw material is adsorbed at the time of precipitating aluminum hydroxide) so as to remove it. A chelating agent is then added to the sodium aluminate solution from which the impurity has been removed, and a carbon dioxide gas is introduced to precipitate aluminum hydroxide which is then washed in an aqueous solution containing the chelating agent to produce aluminum hydroxide.
However, the aluminum hydroxide obtained by this method is crystalline aluminum hydroxide which differs from an aluminum hydroxide gel (amorphous aluminum hydroxide) which is targeted by the present invention. This document proposes that a complex should be formed by adding a chelating agent to sodium aluminate so as to reduce the Fe content of the obtained aluminum hydroxide. However, as the sodium aluminate aqueous solution is alkaline (pH of not less than 10), it is easily assumed that Fe is not existent as an ion but as a hydroxide (super fine particle). Since objective substance must be an ion in a complex formation using chelating agent, aluminum hydroxide having a low Fe content cannot be produced by this method. Further, a complex cannot be produced by the method of washing with water containing the chelating agent, since Fe to be targeted is existent as a hydroxide.
Patent Document 2 proposes a method in which an aqueous solution of supersaturated sodium aluminate having a content of dissolved Na2O of not less than 100 g/l and a Fe content in the liquid of not more than 0.4 mg/l and a molar ratio of dissolved Na2O to dissolved Al2O3 of 1.6 to 2.0 is used as a raw material solution. That is, Patent Document 2 discloses a method for obtaining aluminum hydroxide by controlling the temperature of the raw material, the content of the dissolved Na2O and the above molar ratio, limiting the BET specific surface area of seed crystal aluminum hydroxide to 1 to 7 m2/g and Fe to be contained in the seed crystal aluminum hydroxide and further controlling the supersaturation of the raw material solution. However, even aluminum hydroxide obtained by this method is also crystalline aluminum hydroxide and not an aluminum hydroxide gel particle (amorphous aluminum hydroxide) which is targeted by the inventors of the present invention.    (Patent Document 1) JP-A 2008-19158    (Patent Document 2) JP-A 11-278829